Get all custom properties of an object
UI defined custom properties.
If you mean those defined by the UI

Example, two custom properties on default object
Iterate over custom properties without searching?
Their names via keys
&gt;&gt;&gt; C.object.keys()
['prop', '_RNA_UI', 'prop1']

or via items to get key, value pairs
&gt;&gt;&gt; for k, v in C.object.items():
...     print(f'ob[&quot;{k}&quot;] = {repr(v)}')
...     
ob[&quot;prop&quot;] = 1.0
ob[&quot;_RNA_UI&quot;] = &lt;bpy id prop: owner=&quot;OBCube&quot;, name=&quot;_RNA_UI&quot;, address=0x7fa661ad48c8&gt;
ob[&quot;prop1&quot;] = 1.0

The _RNA_UI member holds all the info re min, max, description for layout, when not defined with a bpy.props. (If defined via bpy.props and non default the data is saved as a custom prop of same name)
Finding the source for &quot;API Defined&quot; custom properties
When should custom-properties be used instead of &#39;bpy.props&#39;?